This invention relates to ultrasonic wire bonding machines; and more particularly, it relates to such machines that are suitable for bonding wires to cavity-down integrated circuit packages.
Traditionally, integrated circuit dies have been packaged in cavity-up packages. A cross section of a typical cavity-up package is shown in FIG. 1A. There the package is indicated by reference numeral 1; and the die is indicated by reference numeral 2. Input/output leads 3 extend from a surface of package 1 that is opposite the cavity in which die 2 is disposed. Electrical signals on the leads 3 are transferred to the die 2 by bonding wires 4.
It is conventional to bond each of the wires 4 between a metal pad on die 2 and a metal pad on package 1 by an automatic ultrasonic bonding machine. Such machines are manufactured, for example, by K & S Corporation of Horsham, Pa. and Mech-El Corporation of Woburn, Mass. They include an elongated wedge 5 and a transducer 6 which together hold a bonding wire 7 and bond the wire to the metal pads.
During the bonding operation, wire 7 is first bonded to the metal pad on die 2. This is achieved by holding the wire 7 between the metal pad and wedge 5 as illustrated in FIG. 1A; and simultaneously vibrating the wedge by means of transducer 6 at ultrasonic frequencies to heat the wire at the wedge. Thereafter, wire 7 slides through wedge 5 and transducer 6 as the wedge 5 is aligned with the metal pad on package 1. Then, a second bond is made between wire 7 and the metal pad on package 1. Subsequenty, the wire 7 is broken by a clamp 8 which closes on the wire and pulls the wire along its axis. Then the entire bonding operation is repeated.
Throughout the above described bonding operation, the angle which wire 7 makes with wedge 5 is fixed. That angle is determined by the position of a cylindrically-shaped hole 5A in the tip of wedge 5 and another hole 6A in transducer 6 through which the wire 7 slides. Typically, hole 6A is located such that wire 7 makes an angle of 60.degree. with wedge 5. Such a large angle is desirable because it insures that wire 7 bends beneath wedge 5 when it makes contact with the metal pad on the die 2. As that angle decreases, wire 7 tends to push out of hole 5A rather than bend under the wedge, which results in a weak bond.
A problem, however, with the cavity-up package 1 is that it is severely limited in its heat-dissipating capability. This is because the leads 3 extend from the surface of the package which lies over the printed circuit board into which the package is to be mounted. Thus, a heat sink cannot be attached to package 1 to cool it.
To overcome this problem, the cavity-down package 1' of FIG. 1B has recently been developed. In it, the leads 3' extend from the surface of the package that has the cavity in which die 2' is disposed. This enables a heat sink 9 to be attached to the opposite surface of the package. Thus, the cavity-down package is suitable for the most modern die 2' which are larger, contain more circuitry, have more input/output leads, and dissipate more power than the traditional die 2.
However, a problem with the cavity-down package is that conventional ultrasonic wire bonding machines cannot be used to bond the wires 7 between the pads on chip 2' and the pads on package 1'. This is because, as FIG. 1B shows, the leads 3' on package 1' interfere with the bonding wire 7 when it is held by the wedge 5 and the transducer 6. Also, the leads 3' interfere with the clamp 8. Either interference will cause wire 7 to break and is simply unworkable.
Accordingly, a primary object of the invention is to provide an improved machine for bonding wires between an integrated circuit die and an integrated circuit package.
Another object of the invention is to provide a mechanism for bonding wires between the pads of an integrated circuit die and the pads of a cavity-down integrated circuit package.